German A2 Level Question

The sentence 'Sie ist eine arme Frau.' correctly uses 'arm' to describe a poor woman. 'Arm' meaning 'poor' is correctly used as an adjective before a noun. Other sentences use 'arm' incorrectly: 'Arm' is not a suitable adjective for 'Auto' (car) or 'Hund' (dog), and the first sentence misuses 'Arm' as a noun (arm), not as an adjective.